1. Field
Aspects of the present invention generally relate to a medium voltage hybrid multilevel converter and a method for controlling a medium voltage hybrid multilevel converter.
2. Description of the Related Art
Traditionally, multilevel power converters are used in the applications of medium voltage AC drives, flexible AC transmission systems (FACTS), and High Voltage DC (HVDC) transmission systems, because single power semiconductor devices cannot handle high voltage. Multilevel converters typically include a plurality of power cells for each phase, each power cell including an inverter circuit having semiconductor switches that are capable of altering voltage states or levels of the individual cells. Depending on the type of inverter circuitry used, e.g., half-bridge or full bridge, each power cell may have one or more switching legs. By controlling switching events of the individual switching legs of each power cell, it is possible to control the voltage across each cell and resultantly obtain an AC output waveform having multiple discrete voltage levels. A multilevel converter is often described by the number of discrete levels in output voltage waveform.
A cascaded multilevel converter is a preferred topology in many medium and high voltage applications. In some cases however, it may require a large number of cells to achieve a desired output voltage level, which contributes to an overall system price increase. An option to solve this problem is to increase the voltage level of each individual cell, thus reducing the number of cells required for the system to achieve a certain output voltage. One drawback with this approach is that systems built with higher voltage cells are optimized in terms of cell count for certain output voltage levels, but they may not be optimized for other output voltage levels.
Cascaded multilevel converters are generally built with identical cells, i.e. same cell topology with the same voltage and same current ratings. A different approach is to use cells with different topologies and different voltage and current ratings, the so called hybrid topologies. In this case, typically the cell with the higher voltage is switched at low (fundamental) frequency while the cells with the lower voltage ratings are switched at higher frequency. This traditional approach suffers from two drawbacks: the high voltage cells will experience unequal losses among themselves and depending on the modulating approach and load conditions, the low voltage cells may have to absorb excess energy from the higher voltage cell.